Most water-soluble polymers containing acrylamide have been widely used as paper strengthening agents, water-purification coagulants, dispersing agents, and treating agents in various other applications.
Various methods have been used to polymerize water-soluble monomers containing acrylamide. Among them, the aqueous solution polymerization method has been used in various fields because it is less expensive and convenient and can easily provide a polymer having a high molecular weight which is essential for the coagulant.
In a polymerization method using a solvent or a dispersion medium, such as solution polymerization, the concentration of a monomer or polymer solution is preferably made as high as possible from the standpoints of the capacity of the polymerizing apparatus and removal or drying of the solvent from the resulting polymer solution. However, when solution polymerization is carried out with a high concentration of monomers, other problems arise. That is, the solution is very viscous, which state prevents the substantial stirring required to remove heat of polymerization by contact of the solution with a cooled surface and to maintain a uniform polymerization temperature. Further, due to the very high viscosity of the resulting polymer, removal of the polymer from the polymerization vessel is difficult.
For example, in the production of a high molecular weight polymer capable of being used as a coagulant from a hydrous polymer containing acrylamide, when the concentration of the starting monomer is 10%, or the concentration of the starting monomer is 30%, the polymerization system is already at a viscosity greater than 1,000 cps, ordinarily of 2,000 to 3,000 cps when the concentration of a polymer in the polymerization system becomes 1%, i.e., at an extent of polymerization or conversion of about 10% and about 3.3%, respectively. Under such a high viscosity, a uniform and effective stirring is no longer possible. If the resulting polymer has a super high molecular weight and (or) a high concentration, a raw rubber-like gel having high viscoelasticity is produced. Therefore, it is impossible to discharge and recover the resulting polymer solution from the polymerization vessel as is ordinarily practiced in conventional solution polymerization.
Under these circumstances, a hydrous polymer containing acrylamide produced by the conventional aqueous solution polymerization is expected to have a non-uniform distribution in the degree of polymerization due to non-uniform temperature control. In fact, a conventional high molecular weight polymer containing acrylamide contains some insoluble portions which are considered to result from a partial ultra super high molecular weight polymer, constituting a serious defect for a paper making paste.
Another problem is that even if the polymer produced can be removed from the polymerization vessel, a large amount of the polymer sticks to the inner wall surface of the vessel due to its stickiness and adhesiveness to the wall thereof and other parts. That is, in the case where the resulting polymer solution has a water content as high as 95 to 90%, the polymer solution can be handled as a fluid. However, when the polymer solution is discharged from the polymerization vessel, a large amount of the polymer solution sticks to the inner wall surface of the vessel and remains thereon.
The residue of such a highly viscous polymer solution sticking to the inner wall surface of the polymerization vessel should be avoided for the following points:
i. When the residue of the polymer is washed away prior to the further charge of an aqueous solution of monomers, it requires a large amount of water and a prolonged period of time because the polymer residue is highly viscous.
ii. When an aqueous solution of monomers is charged without washing away the polymer residue, the remaining polymer is dissolved into the monomer solution, increasing the viscosity thereof. Consequently, the operation of expelling of oxygen dissolved in the aqueous monomer solution, which is necessary for a polymerization operation, becomes difficult to effect, and a uniform dispersion of polymerization initiators is inhibited, which leads to the formation of hot spots.
iii. The polymer residue which could not be dissolved in the aqueous solution of monomers absorbs the aqueous solution and swells. The swollen polymer, when subjected to the subsequent polymerization temperature history, may produce a polymer for inferior in solubility. This is because the swollen polymer will probably give rise to the formation of a super high molecular weight polymer or a cross-linked polymer.